Dereddened NLR ratios of [O III]/Hβ versus R_OIII for the same observational dataset as in Fig. 4. The blue solid line represent the sequence of MB models in which the frontal densities varied from up to 10⁷ cm⁻³, in steps of 0.5 dex, assuming in all cases U_o = 0.1. The gray dotted line represents photoionization models that assumed the same parameters as the MB sequence except that the models are ionization-bounded. In both sequences the cloud internal density is stratified due to the pressure exerted by absorption of the ionizing continuum. The purple line represents the density sequence for the IB component in which the ionizing continuum consisted of the radiation (further diluted by a factor 20) which leaks out from the back of the MB clouds. In all calculations, the assumed ionizing continuum corresponded to the thermal distribution of Fg97 with T_cut = 10⁶ K. The eight (near vertical) short-dash lines correspond to a linear combination of the MB model of density with the IB model with density . The proportion between the two components was set by the parameter A_M/I, which successively took on the values of 10, 3, 1, 0.3 and 0.1. A circle identifies models with A_M/I = 3. To avoid confusion due to the close superposition of the low density sequences, the A_M/I sequences with and 10^2.5 cm⁻³ were left out.